The purpose of a cooling system is to remove heat from a process and discharge that heat to the environment through evaporation and liquid discharge. An example of such systems are cooling water systems used to remove heat from a turbine-condenser. Removing the heat causes steam to condense, resulting in a pressure reduction on the steam-condensate side of the condenser, and an increase in the amount of power generated per unit of fuel used to produce the steam. Similarly, another example of such a system is a refrigeration machine. Such a machine may utilize a refrigerant, such as R-11, R-12 or R-134A, to extract heat from a chilled medium, such as water, loop and transfer the heat to a cooling system.
Another example of a cooling system is one in which heat from a process or series of processes may be transferred to the cooling medium through a process heat exchanger, such as an intercooler on an gas compressor. The operating efficiency of a multi-stage gas compressor, such as an oxygen or nitrogen compressor, may be directly tied to the temperature and pressure of the gas stream entering each stage of the compressor. The higher the temperature and pressure above design conditions, the more power will be required to compress the same amount of gas.
According to one example of a cooling system, the cooling medium includes a liquid, such as water. Such a cooling system may comprise (1) at least one heat exchanger for removing heat from one or more processes; (2) at least one pump for circulating the cooling medium through the at least one heat exchanger; (3) means for cooling the cooling medium, such as a cooling tower; (4) means for adding new cooling medium to the system, such means is typically referred to as makeup; and (5) means for discharging a certain amount of cooling medium from the system, such discharge is referred to as blowdown.
The cooling effect according to such a system may be achieved by evaporating a fraction of the cooling medium into a stream of air as the cooling medium passes over the cooling tower. During the process of removing heat from the cooling medium, unsaturated air may enter the cooling tower. The unsaturated air may pick up the evaporated fluid and exit the cooling tower at a higher temperature and in a saturated state with respect to the cooling medium.
According to another example of a cooling system, a cooling medium, such as water, may be obtained from a large reservoir of the medium that will provide an adequate supply of the cooling medium at a temperature low enough to absorb heat from a process or processes via at least one heat exchanger. In such an example, the cooling system may include (1) a source for the cooling medium; (2) means for circulating the cooling medium, such as at least one circulating pump; (3) at least one heat exchanger; and (5) a place to discharge the heated cooling medium, such as the same body of cooling medium as the source.
Cooling systems that employ liquid cooling media may be subject to deposition of foulant materials and processes, such as, hardness salts, corrosion products, biomass, silt, and mud from the cooling medium, various process leaks, and in-situ corrosion. These foulant materials and processes may be input into the cooling systems from a variety of sources.
Certain treatment materials may be added to the cooling medium or cooling system to inhibit, among other things, hardness deposition, corrosion, formation of biomasses, and agglomeration of other foulants such as silt, mud, corrosion products and process leaks. Such treatment materials may include one or more chemical components that, in combination, effectively inhibit one or more of the above mentioned problems. For example, a treatment material designed to inhibit corrosion may include at least one cathodic inhibitor, at least one anodic inhibitor, and/or at least one additional material, such as anti-scalant(s), surfactant(s) and anti-foam agent(s). Other treatment materials that may be added to the system/medium may include one or more acids, such as sulfuric acid, or one or more alkaline materials, such as a solution of caustic soda. These additional materials may control the pH of the cooling medium within a predetermined control range. When controlling the pH, the acid would be used to lower the pH and the alkali to raise it.
Typically, the amount of treatment material(s) added to the system/medium may be determined by manual laboratory tests, corrosion coupon analyses, the volume of cooling medium in the system and the amount of cooling medium supplied to the system in a given period of time. In such cases, means, such as a chemical injection pump, may be used for injecting a flow of a treatment material into the system at a constant rate over time.
When adding material(s) to adjust pH, a closed loop chemical injection system may be utilized. For example, the system pH may be monitored by a pH monitoring device that controls a pump or valve to increase or decrease the feed of the pH adjusting agent(s).
Treatment materials used to inhibit the deleterious effects of process leaks are typically controlled manually, either on a continuous basis or upon detection of the occurrence of such a leak. Detecting, locating and quantifying a process leak is a time consuming task that may be performed by a plant operator or treatment professional.
Another important aspect of treatment and control of a cooling system and the quality of the cooling medium relates to maintaining the concentration of dissolved and suspended materials in the medium below the level at which they may precipitate or agglomerate. This is typically accomplished by adjusting the amount of concentrated cooling medium removed from the system via blowdown. Typically, this has been done by manual adjustment of a blowdown valve, based on laboratory analysis of the circulating cooling medium. More recently, blowdown has been automated by monitoring the specific conductivity of the cooling medium and adjusting a blowdown valve to maintain a specified range of conductivity within the system.
Typically, a cooling system includes a cooling medium, such as water. The characteristics or properties of the cooling medium may affect the cooling system and its operation. Cooling medium parameters such as pH, conductivity, corrosion rates, temperatures (such as cold supply, hot return, heat exchanger inlet, and outlet, among others), cooling medium recirculation rate, cooling medium make-up rate and cooling medium blowdown rate have long been monitored to control the characteristics of cooling medium. Blowdown may serve to deconcentrate the dissolved solids in the cooling medium and the makeup replaces all cooling medium losses, including those from evaporation, windage, drift and blowdown. Historically, automated control of cooling systems has been based upon on-line measurements of conductivity and pH, for example. Measurements of these parameters may be supplemented by laboratory wet chemistry tests for residual inhibitor level and other parameters relevant to the specific application.
The use of linear polarization resistance technology for on-line measurement of corrosion rates was previously disclosed in U.S. Pat. Nos. 3,069,322, 3,156,631, 3,250,689, 3,607,673 and 3,698,065, the entire disclosures of all of which are hereby incorporated by reference. In U.S. Pat. No. 4,339,945, to Knudsen, and U.S. Pat. No. 4,346,587, to Knudsen et al., the entire disclosures of both of which are hereby incorporated by reference, a methodology was disclosed where in a device in which the measurement of fouling added to pH, conductivity and corrosion is disclosed and applied to determine the effectiveness of a treatment regime in inhibiting corrosion and fouling in fluid systems, such as cooling water. Since corrosion may decrease the economic life of equipment in the cooling system and fouling may reduce the efficiency of heat rejection from plant processes to the cooling medium, it is very important to keep these under control.
Traditional control of open recycling cooling systems typically includes the use of a single loop controller for controlling the pH with either an acid, such as sulfuric acid, or a base, such as a solution of caustic soda. A single loop controller may be included for controlling the level of dissolved solids in the cooling medium. Such dissolved solids may be measured by specific conductivity, and may be increased or decreased via adjusting a blowdown, or bleed off valve. Increasing the rate of bleed off causes the dissolved solids in the cooling medium to decrease. Some type of feed control for treatment chemicals may also be utilized for controlling the characteristics of cooling systems. Calendar date and time combined with an interval timer function are commonly utilized in control of systems and, in particular, for adding various types of microbicides.
Control of pH and conductivity typically utilizes feedback control technology. Feedback control may provide either analog or digital outputs to operating equipment such as pumps and valves. Analog outputs may go through Proportional Integral Derivative (PID) controllers for more precise control. Make-up medium flow rate, the flow rate of cooling medium added to the system compensate for medium lost to evaporation and other losses, such as windage and blowdown, is typically the basic control parameter for feed of treatment chemicals such as inhibitors and dispersants. In a cooling system, and an aqueous cooling system in particular, the make-up medium may be water supplied from at least one source, such as potable municipal water, well water, reclaimed process water, externally treated water, and condensate. Another commonly used, but less precise, method is to use the blowdown medium flow rate, which requires an assumption that other liquid losses may be either fixed or proportional to blowdown. A third means is called "bleed and feed", which proportions the addition of treatment materials to the time that an on/off, for example, a solenoid, bleedoff or blowdown valve is actuated.
Linear Polarization Resistance (LPR) technology has been used for three decades to monitor corrosion rates on-line. This technology is marketed as a "CORRATER" monitor by Rohrback Cosasco Systems, Inc. The analog signals provided by the CORRATER monitor are proportional to the "instantaneous" corrosion rate or the "instantaneous" corrosion imbalance. In fact, the CORRATER monitors determine the LPR over a period of time that may be as little as about 2 minutes or as long as about 20 minutes. Thus "instantaneous" is a relative term when compared with off-line techniques, which may require about 30 to about 90 days of coupon exposure and as compared to on-line electrical resistance techniques, which may require about 2 to about 30 days.
Fouling Factor (FF) is a recognized engineering unit of measure. The FF results from the deposition of foulants on a heat transfer surface. The P-U-L-S-E analyzer, available from Ashland Chemical, Drew Industrial Division, provides the ability to determine the FF. The ONGUARD CFM-1000 monitor, also available from Ashland Chemical, Drew Industrial Division, determines a Fouling Index (FI). Research has proven that FI and FF have identical trends, yet they are numerically different and are computed in different manners.
Known methods of monitoring characteristics of cooling systems and controlling parameters of the systems may be of three types. The first type of include those that have traditionally lent themselves to automated feedback control of certain parameters, such as pH for the control of acid or caustic feed and conductivity for the control of bleedoff or blowdown.
The second type are those that detect the residual of a specific component in the treatment chemistry where such can be adequately measured in such a stream of the cooling medium. There are two known approaches to this type of monitoring. The first approach includes detecting a single active component, while the second type of detecting includes detecting an inert material that acts as a tracer.
Phosphate monitors are examples of the first approach to detecting since they detect only a single species of phosphorous ion, orthophosphate, in the cooling water system. In fact, the treatment chemistry may include several different phosphorous bearing materials together with various polymers, aromatic azoles and other constituents.
In the presence of calcium, magnesium, ferrous and ferric ions, orthophosphates have limited solubility. The limited solubility is a complex function of pH, temperature, residence time, and ionic concentration. In some systems, the solubility of such materials under operating temperatures and pressures may be close to the desired control limits making it difficult to differentiate between an excessive feed and conditions that may lead to deposition and, ultimately, to under deposit corrosion, which, in addition, leads to control windup.
Use of an inert trace compound that, unlike the active materials, can be detected in the stream of the cooling medium has its own shortcomings because it may only tell the user how much trace material is in the system. It may be unable to determine the efficacy of the treatment. For example, a treatment material, such as active constituents in a corrosion inhibiting composition, typically are consumed by reaction(s) in the process of inhibiting corrosion, or whatever process they are directed to inhibiting. The remaining treatment material is that portion that the reaction(s) do not consume and/or residual portion(s) that are required to sustain the involved reaction(s) by driving them to completion. Inert material(s), on the other hand, pass through the system without reacting with anything. This dependence on trace components does not assure the maintenance of active components within the desired control limits to assure that the performance benefits of said active components are derived. The above discussion may apply to any treatment material.
Other monitoring techniques include off-line monitoring such as complete chemical and microbiological analysis of the circulating water and analysis of corrosion coupons.
Many automated chemical feed systems utilize closed loop feed forward logic. These approaches use measured variables such as flow rate changes and changes in pressure, such as a pressure drop. The chemical feed quantities are adjusted through mass balance based algorithms associated with these measured variables. Further improvements were made in the accuracy of the mass balance algorithms by utilizing secondary measurements of conductivity and pH. However, these chemical feed systems only assured chemical mass loading maintenance within the system and did not take into account losses of the additives due to reactive consumption, precipitation or adsorption.
Other technologies were developed to feed chemicals based upon detection of quantifiable traceable entities associated with the chemical fed. While this approach alone ensured chemical quantity presence over the preceding method or in conjunction with preceding method whereby the method was utilized as a feed back trim. Approaches to date have not ensured the maintenance of key performance variables. For example, the key performance variables in cooling water systems are corrosion and fouling.
The minimization of the degree of corrosion and fouling in cooling waters are dependent upon the site-specific make up water characteristics, system design and operational criteria. As a result, a plurality of chemicals may be required to maintain a cooling system's corrosion and fouling control. Chemicals such as and not limited to ferrous and non-ferrous corrosion inhibitors, scale control agents, dispersants for inorganic and organic foulants, oxidizing and non-oxidizing biocides, biodispersants as well as specialized contingency chemicals to handle chemistry upsets due to process side ingressors. Should all these additives contain a tagged tracer to the active ingredient(s) or an inert tracer proportional concentration of the active ingredient(s), they could be separated individually for quantification as to the amount of each present in the system water utilizing on-line monitoring techniques, such as calorimetric, U.V., fluromatic, among others.
However, with such feed back trims in place, there still exists a lack of linkage of chemical(s) addition to the key performance variables, which are impacted by the chemicals. Examples of such systems include the system disclosed by Canadian Patent 2,055,098, U.S. Pat. Nos. 5,006,311, 4,992,380 and 4,783,314, which illustrate the process of detection of tracer tagged additives. These online detection methods then permit an increased, below specification, or decrease, above specification, in dosing rates of the detected additive. Should the additive be depleted from process inleakage or sludge adsorption for example, then continuous overfeed of the additive would occur. The residual loss and/or maintenance are not deriving any key performance value as it is not part of additive's designed intended use. Thus, control wind up results in an uneconomical consumption of the additive which in fact could impact the key performance variables negatively, such as increased fouling and/or corrosion.
Another example of such a system is disclosed by Rohm and Haas Company of Philadelphia, Pa., who have refined the traced tagged polymer approach as outlined in their literature from 18358B, Bulletin FC415. The method revealed therein indicates that free/unreacted polymer rather than total polymer can be detected by laboratory bench testing means. Unfortunately, this too can lead to control wind up whereby if system operational design or process inleakage occurs, the corrosion inhibitor, phosphate, will be lost in heat exchanger surface deposition, necessitating increased phosphate and tagged polymer addition. Not only is fouling increased but also underdeposit corrosion is initiated. This method again is not linked to key performance parameters of the system being treated. Only the economics associated to overfeed are reduced slightly over the previous citing and at present the method cannot be performed on-line and coupled to automatic chemical feeding as the former.
Yet another system is disclosed by U.S. Pat. Nos. 4,966,711 and 5,200,106, which teach the use of inert transition metals for tracers. These tracers are added in proportion to a chemical additive package. Should the additive(s) be consumed or lost due to coprecipitation or adsorption fouling then the tracer would indicate sufficient additive is present when in fact the opposite is true. This then results in control wind down. Corrosion protection can then be compromised. The key performance parameters of the system are not addressed.
Ion selective electrodes can and have been utilized to detect both inert tracers and corrosion inhibitors, such as phosphate. Inert tracer detection can lead to control wind down, while corrosion inhibitor detection of say phosphate can lead to control wind ups as previously mentioned.
U.S. Pat. No. 5,272,346 disclosed that certain corrosion control additives which are sensitive to U.V. absorption, such as imidazoline and pyridine derivatives, can be monitored on-line. However, no attempt in correlating the key performance parameter of corrosion rate determination and the impacted detectable amounts of the corrosion control additive have been disclosed. Should the relationship be established, again there is no assurance that an increase or decrease in additive concentrations would produce the desired result of improvements in the corrosion control as other foulants or conditions may be impacting the on-line corrosion rate being monitored.
Utilization of traced non-oxidizing biocides has also been proposed by Cooling Tower Instituted of Houston, Tex., Cooling Tower Water Chemistry Paper T.P. 95-16. This approach allows the user to ensure that a residual amount of biocide additive in the system water over the period of desired contact time is correct. It does not relate to the control of the microbiological population, such as efficacious kill rates, and more importantly is not related to the detection and mitigation of biological fouling or microbiological influenced corrosion. Again, the key performance parameters are not addressed, which are, in fact, related to the need and use of the biocidal additives if smart logic can be used for the on-line identification of the use need.
The above discussion reveals that such approaches, at best, may permit a user to cross check chemical feed(s) based on mass balance, feed forward, and trim the specified amounts based on the on-line detection of the actives, feedback. This may allow for the potential of chemical inventory and economic control in a dynamic perfect system. Under upset or loss conditions, there is a potential for poor economics.
According to one analysis, the approximate operating costs of a cooling system, excluding downtime opportunity costs, may be broken down as follows:
______________________________________ Percentage of Total Cost Category Operating Costs ______________________________________ Electricity 59 Equipment Depreciation 17 Labor 14 Maintenance 5 Water Sewerage 3 Water Treatment Management 2 ______________________________________
The above-discussed approaches may impact the water treatment management portion and allow a user some ability to optimize the 2% of the cooling system cost. However, without linkage to the key performance parameters the remaining 98% of the operating costs cannot be effectively impacted. It is also well known that ineffective water treatment can lead to an increased operating cost of 4 to 5 times the cost of effective water treatment in simple refrigeration cycle cooling systems. Clearly, there is a need for a control system, which is linked to site specific key performance parameters through smart logic.